Self-expanding implants such as stents and stent grafts are often delivered to a stenting site within a bodily lumen with the use of a catheter delivery system that is advanced percutaneously and transluminally. Although most stents and stent grafts are for the cardiovascular system, self-expanding implants can also be delivered transluminally to body lumens that carry bodily fluids other than blood. A stent without a coating is often called a “baren stent. Stent grafts that carry a covering of a material such as expanded polytetrafluoroethane (ePTFE) are often called “covered” stents or “stent grafts”. A self-expanding stent need not be made of metal but usually is, and that metal is usually a nickel titanium shape memory alloy commonly known as “NITINOL”.
Given that a self-expanding stent will expand when freed of the constraint of the catheter delivery system, it follows that the catheter delivery system confining the stent will be subject to radially outward pressure from the confined stent, at least at body temperature 37° C. With NITINOL, the outward radial pressure dwindles to zero as the temperature of the stent is reduced to temperatures around 0° C. and below 1 with the austenitic crystal lattice changing, as the temperature reduces, to a martensitic crystal lattice.
Thus, at low temperatures, with the self-expanding stent in the martensitic state, the hoop stress on the sheath surrounding the stent in the delivery system will be relatively low, even to the extent of being close to zero. However, as the temperature rises towards body temperature, the radially outward pressure on the confining sheath will increase. Given that the confining sheath has to be flexible if the distal end of the catheter delivery system is to advance along a tortuous bodily lumen, it is invariably made of a synthetic polymeric material rather than metal. Such materials are subject to deformation and the deformation of polymers is a time-dependent phenomenon. Suppose that the self-expanding stent confined within its sheath is stored for a period of weeks or months/at room temperature or above. There is the possibility/perhaps likelihood, that the sheath will stretch and the stent will expand radially to some extent/during the extended period of storage.
Even more significant, in coated stents such as those made of Nitinol with an ePTFE covering, relaxation of the compacted ePTFE layer on the stent also contributes to radial distortion of the sheath.
As the quest continues for ways to deliver implants to ever-smaller diameter locations within the body, through ever-more tortuous delivery paths, the pressure on designers of implants and delivery systems to reduce to ever-smaller values the passing diameter of the distal end of the catheter system where the implant is located, continues to increase. This pressure pushes designers to think of sheath designs of ever-smaller wall thickness. The smaller the wall thickness of the sheath, the greater the difficulty of resisting the radially outward pressure imposed on the sheath by the stored implant.